Intel 253666-024US Manuel D’Utilisation

3-312 Vol. 2A

FCOS—Cosine

INSTRUCTION SET REFERENCE, A-M

FCOS—Cosine

Description

Computes the cosine of the source operand in register ST(0) and stores the result in 
ST(0). The source operand must be given in radians and must be within the range −

2

63

 to +2

63

. The following table shows the results obtained when taking the cosine of 

various classes of numbers.

If the source operand is outside the acceptable range, the C2 flag in the FPU status 
word is set, and the value in register ST(0) remains unchanged. The instruction does 
not raise an exception when the source operand is out of range. It is up to the 
program to check the C2 flag for out-of-range conditions. Source values outside the 
range −2

63

 to +2

63

 can be reduced to the range of the instruction by subtracting an 

appropriate integer multiple of 2π or by using the FPREM instruction with a divisor of 

2π. See the section titled “Pi” in Chapter 8 of the Intel® 64 and IA-32 Architectures 

Software Developer’s Manual, Volume 1, for a discussion of the proper value to use 
for π in performing such reductions.
This instruction’s operation is the same in non-64-bit modes and 64-bit mode.

Operation

IF |ST(0)| < 2

63

THEN

C2 ← 0;

Opcode

Instruction

64-Bit 

Mode

Compat/

Leg Mode

Description

D9 FF

FCOS

Valid

Valid

Replace ST(0) with its cosine.

Table 3-28.  FCOS Results

ST(0) SRC

ST(0) DEST

−

∞

*

−

F

−1 to +1

−0

+1

+0

+1

+F

−1 to +1

+

∞

*

NaN

NaN 

NOTES:

F Means finite floating-point value.
*  Indicates floating-point invalid-arithmetic-operand (#IA) exception.